The invention relates to an MgB2 single crystal having anisotropic superconductive properties, a superconductive material containing the same, and a method of manufacturing the same, making it possible to provide a superconducting wire and superconductive thin film that are operatable at a relatively high temperature.
Since it has recently been discovered that MgB2, that is, magnesium boride is a superconductive substance having a relatively high critical temperature (Tc =about39K), researches from various viewpoints have been conducted in order to elucidate its properties in detail. However, an MgB2 material has been so far known only in the form of xe2x80x9cfine powdersxe2x80x9d or xe2x80x9cpolycrystalline bulk materialxe2x80x9d, and further, there has been reported no case concerning manufacture of an MgB2 single crystal, so that satisfactory results of the researches have not been gained as yet.
A reason for difficulty encountered in growing a single crystal of MgB2 lies in that, upon heating MgB2 in an attempt to obtain first MgB2 melt in order to cause growth of the single crystal thereof, there occurs a phenomenon causing MgB2 to be decomposed into MgB4 and MgB6 at a temperature lower than the melting temperature of MgB2.
Although there has been a report stating xe2x80x9ca trace of MgB2 that appeared like crystallites was detected as a byproduct upon synthesizing cubic boron nitride (cBN) from hexagonal boron nitride (hBN) under high pressurexe2x80x9d (N. E. Flonenko et al. Dokl Akad. Nauk SSSR 175 (1967), pp. 833 to 836), details of the report are unknown, being far from contributing to elucidation of the properties of MgB2 in detail.
Under the circumstances, it is an object of the invention to establish means for manufacturing MgB2 single crystals, thereby opening a way to considerably expand applicable fields of MgB2 that is highly hoped for as an excellent superconductive material.
To that end, the inventor et al. have conducted intensive researches, and as a result, the following knowledge has been obtained.
(a) As previously described, since MgB2 is caused to be decomposed into MgB4 and MgB6 before MgB2 is melted if the same in as-is state is heated at a high temperature, it has been impossible to obtain stable MgB2 melt. However, if a mixed raw material of Mg and B or MgB2 powders obtained by causing reaction of the mixed raw material of Mg and B, kept in contact with hexagonal boron nitride (hBN), respectively, is heated up under high pressure, this will generate Mg3BN3 that is a eutectic composition of Mg, B and N, and so forth, whereupon there occur regions where Mg3BN3 and so forth are turned into melt thereof at a temperature lower than the decomposition temperature of MgB2 in an Mg-B2 constituent system, and if this state is kept, there occurs crystallization of MgB2 crystals from the melt containing Mg3BN3 and so forth, thereby causing crystal growth of the MgB2 crystals without occurrence of decomposition into MgB4 and so forth.
(b) In this case, if the MgB2 powders in non-melted state exist in the melt, these will act as nuclei for crystallization of the MgB2 crystals, thereby causing crystal growth thereof, so that it becomes possible to grow relatively large crystals in shorter time.
(c) Further, if a reducing agent (such as Mg etc. having strong oxidizability) is caused to coexist in a reacting system in such a way as to be spatially separated from the mixed powders of Mg and B that are raw materials of MgB2 crystals, the reducing agent will absorb oxygen that cannot be prevented from being mixed into the reacting system, thereby lowering a partial pressure of oxygen in the reacting system, so that growth of the MgB2 crystals is facilitated, resulting in more stable growth of MgB2 single crystals.
(d) In addition, if a temperature gradient is caused to occur in Mg3BN3 melt occurring by heating up under high pressure, this will further promote the growth of the MgB2 crystals.
(e) The MgB2 single crystals obtained by the above-described method, and so forth, are of a hexagonal structure wherein two-dimensional boron atom layers and magnesium atom layers are alternately deposited on top of each other in the vertical direction, exhibiting pronounced anisotropy in respect of superconductive properties (for example, a second magnetic field Hc2 in the case where a magnetic field is applied in such a way as to be parallel with boron faces thereof differs considerably from that in the case where the magnetic field is applied in such a way as to be perpendicular to the boron faces), so that it is possible to manufacture a kind of material capable of exhibiting superconductive properties of MgB2 in the best state by forming a configuration wherein a plurality of the single crystals are bonded in such a way that respective crystal orientations thereof are aligned with each other.
(f) The lower the temperature of the MgB2 single crystals is, or the higher the purity and crystallinity thereof are, the more pronounced the anisotropy in respect of superconductive properties of the MgB2 single crystals becomes, however, with the method described in the foregoing, it is possible to obtain such high-purity MgB2 single crystals as one having xe2x80x9can anisotropy ratio not less than 2.3 at a temperature of 25Kxe2x80x9d, and in addition, this MgB2 single crystals have a feature such that an irreversible magnetic field Hirr, is very close to a second magnetic field Hc2 in the case where a magnetic field is applied thereto in such a way as to be parallel with the boron faces. Taking advantage of the feature, it is possible to manufacture a superconductive material and so forth that can allow large superconducting current to flow even if a high magnetic filed is applied thereto provided that the high magnetic filed is parallel with the boron faces, thus enabling applicable fields of MgB2 to be considerably expanded.
Herein the anisotropy ratio described above is defined by the following formula       anisotropy    ⁢          xe2x80x83        ⁢    ratio    =                    Hc        2            ⁡              (                  H          //          c                )                            Hc        2            ⁡              (                  H          //          ab                )            
(g) Further, it is not that a kind of material capable of having the above-described superconductive properties based on the MgB2 single crystals is not necessarily limited to material made up of the MgB2 single crystals only, and any material can exhibit correspondingly excellent superconductive properties even if other substances (for example, Mg, B, and MgB2 powders that have not reacted as yet) are mixed therein provided that the MgB2 single crystals are included therein.
The invention has been developed on the basis of respective items of the knowledge described above, and is intended to provide a MgB2 single crystal superconductor, and methods of manufacturing the same, as described under the following items (1) to (8), respectively.
(1) An MgB2 single crystal having anisotropic superconductive properties such that a critical magnetic field anisotropy ratio at a temperature of 25K is not less than 2.3, and in the case where a magnetic field is applied thereto so as to be parallel with boron faces, an irreversible magnetic field strength is equivalent to not less than 95% of a second magnetic field strength.
(2) A method of manufacturing MgB2 single crystals, comprising the steps of:
preparing a mixed raw material of Mg and B; heating and melting the mixed raw material, kept in contact with boron nitride (BN), at a high temperature in the range of 1300 to 1700xc2x0 C. and under a high pressure in the range of 3 to 6 GPa; and
causing growth of the MgB2 single crystals having anisotropic superconductive properties by holding the mixed raw material in the above-described state.
(3) A method of manufacturing MgB2 single crystals, comprising the steps of:
producing a precursor containing MgB2 crystallites, obtained by causing reaction of a mixed raw material of Mg and B;
heating and melting the precursor, kept in contact with hexagonal boron nitride (hBN), at a high temperature in the range of 1300 to 1700xc2x0 C. and under a high pressure in the range of 3 to 6 GPa; and
causing growth of the MgB2 single crystals having anisotropic superconductive properties by holding the precursor in the above-described state.
(4) The method of manufacturing MgB2 single crystals, as set out under the above-described items 2 or 3, wherein in the course of heating, and melting the raw material or the precursor, to be heated and melted, at the high temperature and under the high pressure, and holding the same in the above-described state, a reducing agent is caused to coexist therewith.
(5) The method of manufacturing MgB2 single crystals, as set out under any of the above-described items 2 to 4, wherein in the course of heating and melting the raw material or the precursor, to be heated and melted, at the high temperature and under the high pressure, and holding the same in the above-described state, a temperature gradient of from 150 to 300xc2x0 C. is provided in melt occurring as a result of the heating and melting the raw material or the precursor.
(6) The method of manufacturing MgB2 single crystals, as set out under any of the above-described items 2 to 5, wherein the MgB2 single crystals have anisotropic superconductive properties which are superconductive properties such that a critical magnetic field anisotropy ratio at a temperature of 25K is not less than 2.3, and in the case where a magnetic field is applied thereto so as to be parallel with boron faces, an irreversible magnetic field strength is equivalent to not less than 95% of a second magnetic field strength.
(7) A superconductive material comprising MgB2 single crystals having anisotropic superconductive properties such that a critical magnetic field anisotropy ratio at a temperature of 25K is not less than 2.3, and in the case where a magnetic field is applied thereto so as to be parallel with boron faces, an irreversible magnetic field strength is equivalent to not less than 95% of a second magnetic field strength.
(8) A superconductive wire rod comprising MgB2 single crystals having anisotropic superconductive properties such that a critical magnetic field anisotropy ratio at a temperature of 25K is not less than 2.3, and in the case where a magnetic field is applied thereto so as to be parallel with boron faces, an irreversible magnetic field strength is equivalent to not less than 95% of a second magnetic field strength.
As described in the foregoing, with the invention, stable growth of MgB2 single crystals can be attained by specifying conditions, and so forth, for implementing liquid state of MgB2, so that the MgB2 single crystals or a superconductive material comprising the MgB2 single crystals, hoped for use in wide application fields, can be provided.